Electrode structure for color cathode ray tube



Nov. 11, 1959 H. J. EVANS 2,913,613

ELECTRODE STRUCTURE FOR COLOR CATHODE RAY TUBE Filed Aug. :s, 1956 2 FIG.4. v L @R 11 w 0 m i. =3e o L INVENTORI HOWARD J. EVANS BWMMW ms ATTO 1 1-:Y.

ELECTRODESTRUCTUREFOR COLOR,

CATHODE-RAtY TUBE Howard J. Evans, Fayetteville, N. Y., assignor to General Electric Company, a corporation of New York ApplicatiomAugust@1956, Serial No. l60 1,:9, 20

s claims. cl.---31s-7s The present .in ventionsrelates to improvements in "cathode. ray tubes of the type :wherein an electrode consisting of .a grille of fine spaced linear conductors is -disgrilleof linearconductors,.forexarnple fine wires, is dis-.

2,913,613 Patented Nov. 17, 1959 ice scribed, which efiectively eliminates the electrostatic field distortions heretofore encountered.

Another object is to provide in a tube of the character described an improved grille assembly including a wire vibration damping element having improved secondary electron emission characteristics such as to provide optiposed ,in thepath of .the electron beam ,to the screenand maintained .at such .a potential relative to that of the screen ,as is produce antelectrostatic.-field for ac- ,celeratingelectronsiof theibeamtoward the screen, This .type of tube has utility ,for example asa .colortelevision picture ,tubelwherein the acceleration .of .the .electrons increases .thedntensityof light outputat the screen and wherein the grille also serves as a focusing and color selection electrode providing .accuratecolor reproduction. One'of the problemsassociated withsuchtubes is that of vibration of the .grille conductors ,duringtub'e.

Various arrangements have been suggested togprevent such Vibrations oi the grille wires.

One suggestion -.con

mum compatibility of the damping element with the electrostatic field adjacent the grille.

7 These and other objects of the invention will be apparent from the following description, and the scope of the invention ,will be defined in the appended claims.

Briefly, the foregoing objects are attained in accordance with the present invention by providing a damper fiber-o fcomposite construction, consisting 'of a central ,core of an insulating material having good'mechanical properties so that it may be readily fabricated in filamentary form and easily applied to the grille, and an outer sheath or coating on the core consisting of ama- .;terial having a secondaryelectron emission-versus-voltage characteristic such that the potential to which each portion .of the damperelement surface charges during tube operation is substantially equal to the-space potential .of .the electrostatic field formed by the grille, thus substantially eliminating any'distortion of the field adjacent the damper.

For a more complete description of the invention reference is made to the accompanying drawing wherein:

Figure 1 is a View partly broken away in axial section of a cathode ray tube of a type with which the present invention hasparticular utility.

Figure, 2 is an enlarged sectional View of a fragment of thejgr'ille and screen of the-cathode ray tube of Figure Figure 3 is a graph illustrative of a characteristic variation-i secondary electron emission ratio with accelerattemplates the provision of one or moreinsulating fiberlike vibrationdamping elements interwoven through the grille in transverse relation to the conductors thereof intermediate their'ends. "Such a damping .fiber. must .be a non-conductor in order to avoid creating a substantial discontinuity in thefllectrosltatic field adjacentsthe grille.

However, because fitjfis an insulator, the damping,,element tends (to become 'char-gedby .bor'nbardmen'tlby the electron beams dur'ingoperaftion of they tube, the po-r of ready fabrication .into filam'entary fonn ,and easy "assembly in a grille have the unfortunate property of being chargeable by electron beam bombardment -to va :potential which difiers markedly from a desired grille Wire potential. .The resulting potential difference bctweenthe damper fiber and the'grille conductors produces at-severe distortionyof the electrostatic field adjacent the grille in I the region of the damper fiber and creates "a noticeable and highly objectionable electron shadow of the damper fiber on the screen. Accordingly, one object of the present invention is to provide in aj'cathode raytuhe off the character described an improved damper element for efiectively minimizing vibrations otthe conductors of the grille ,without forming an objectionable electron shadow on the screen.

jing voltage of a material bombarded by primary electrons, Figure 4 is a view similarto FigureZ taken on the line 4' 4 thereof; and

Figure 5 is a sectional view'of 'a damper element-constructed in accordance with the Present invention.

Turning now to thedrawing, a cathode ray tube .of a type "with which the present invention is particularly suitable is shown in Figure 1 and includes an envelope 2 enclosing one or more electron guns 4, there shown for example as three in number, and provided on' the interior of its viewing face with a screen 6 including a layer of luminescent material '8 having a conductive backing layer It). In a cathode ray tube for reproducing information in natural color, the luminescent material 8 may, for example, consist of a plurality of stripe triads,- each triad consistingof three narrow stripes .of screen material each capable of producing light of a different additive primary color when excited by electron bombardment. The electron beam from each gun is scanned across the screenby suitable deflecting-means 12. Mounted in the path of the electron beams, near and generally parallel to the screen. is a grille electrode 14- consisting of an array of parallel spaced fine wires 15. The grille 14 is maintained by a suitable power supply, shown schematically as battery. 16, at an accelerat- "ing potential with respect to thecathodes of the electron guns, and the conductive layer 10 of the screen is maintained at a suitable potential with respect to the grille so as to create an electrostatic field between the grille 'menttor'use incathode ray-tubes of the character'deand-screen. The electrostatic field ishere shown as an accelerating field, but might of course, also bea decel- --erating field. An electron collector anode 18 on the funnel 'wall of1the tube is also maintained at a suitable ,potential, which may,.for example, be slightly above'the grille potential,

the field created by each pair of wires and the space therebetween to act as a cylindrical electron lens for focusing electron beams passing therethrough, while the 'equi-potentials 22 spaced further from the Wires are progressively less curved. In the operation of the tube, various factors such as variations in the voltage supplied 1 from the power supply to the screen and grille and variations in the electron beam current may cause the wires of the grille to vibrate. Such vibration causes periodic -displacement of portions of the grille Wires transversely of their length, which displacements can cause sigmfi- 'cant distortion of the electron beam trace on the screen and may, inVmosaic-screen tubes for reproducing images in naturalcolor, cause objectionable color impurity in the reproduced picture.

To overcome this problem of such grille conductor vibration it has heretofore been proposed to weave a slender fiber through the grille transverse to the con- .ductors 15 thereof so as to shorten their unrestrained length and thereby increase their natural resonant frequency, while providing an effective mechanical damping action. The position of such a fiber is shown at 26 in Figure 1 and Figure 2. The fiber is preferably made 'of insulating material so as not to provide a conductive surface to which flux lines of the electrostatic field, illustrated at 28, may be attracted and thereby cause objectionable distortion of the field. One of the difficulties of such a prior art arrangement however is that those insulating materials, such as various glasses, which are most practical to use for the damper fiber from the standpoint of ready fabrication into filamentary form, ease of insertion into the grille, lack of brittleness, etc., do

not have suitable electrical properties for this applica- Ition. Such materials have the characteristic of emitting secondary electrons in response to bombardment by the primary electrons of an electron beam at a rate such that as damper fibers they will quickly charge up to a potential substantially different from the desired grille potential, and thereby produce a severe distor- -tion of the electrostatic field which shows up as an objectionable shadow or line of contrast on the screen.

For an explanation of this phenomenon reference is.

made to Figure 3 which is a graph of secondary electron emission ratio, ls/Ip, versus accelerating voltage with respect to a bombarding source, Vp, of a target bombarded by primary electrons. As the curve indicates, as the accelerating potential Vp is increased the secondary emission ratio increases from a value less than 1.0 to 1.0 at point 30, then further increases to a value greater than 1.0, and finally at a point 32 it again falls below 1.0. The point 32 is referred to as the second cross over of the curve, and the corresponding value of Vp is referred to as Vp the sticking potential of the material. The term sticking potential is used because it is the potential at which the bombarded material stabilizes when bombarded with primary electrons with an accelerating voltage greater than that at point 32, in the vicinity of another conductor at a higher potential to which it can give off secondary elec trons. No matter how high the nearby collector potential is raised, or how high the accelerating potential is raised, the bombarded material will always charge to,

but not in excess of, its sticking potential. This is because if the bombarded material starts off at a potential less than the sticking potential its secondary emission ratio will be greater than 1.0 and it will give off electrons faster than it receives them and become more positive until it reaches the sticking potential, and if it starts off at a potential higher than the sticking potential it will have a secondary emission ratio of less than 1.0 as indicated in Figure 3, and will therefore acquire more electrons and become progressively more negative until it reaches the sticking potential.

The practical effect of this phenomenon on the damper fiber 26 is as follows: damper fibers of material which it is practical to employ from the mechanical standpoint have sticking potentials which are less than the grille potential which it is desirable to employ in post acceleration tubes of popular size. For example, in a color television picture tube of the size popularly referred to as 21", the grille potential may be approximately 6.5 kilo volts, whereas many available damper fiber materials have a sticking potential of only 4.0 to 4.5 kilovolts. This means that during the operation of such a tube the damper fiber will charge in response to electron beam bombardment to the 4.0-4.5 kv. level, at which it is some 2.5 kv. below the grille potential. This substantial voltage gradient between the damper fiber and the grille wires creates a severe distortion of the space-potential gradient of the electrostatic field between grille and screen -in the vicinity of the damper.

As shown schematically in Figure 4, under such conditions the otherwise uniformly spaced flux lines of the field are attracted by and tend to terminate on the damper fiber 26 because of its substantially lower potential than the grille Wires 15. The flux lines 36 in the neighborhood of the damper fiber are thus curved in toward the fiber while those flux lines 38 more remote from the damper fiber are unaflected. The resulting curvature ofthe flux lines 36 of the field of course produces a corresponding curvature of the path of an electron beam traversing the field, according to the well known laws of electron-optics, and the effect of such distortion of the beam path is to create an electron shadow on the screen which is many times the width of the damper fiber itself, and hence highly objectionable from the viewing standpoint.

To correct this situation it is proposed in accordance with the present invention to provide a damper fiber of composite construction, consisting of a filamentary core of any suitable insulating material having desirable mechanical properties enabling it to be readily drawn into filaments or rods and easily applied to the grille, and a coating on the core of a material having a high enough stickingpotential so that the etfective sticking potential :for the composite fiber is above that of the desired grille potential.

This construction is illustrated in Figure 5, where 42 denoted a core having a coating 44. The

choice of the material for the coating will be affected by a number of considerations, first among which of course is the desired grille potential. Another factor affecting the choice of coating material is the tendency of the sticking potential of most bombarded materials to decrease during the'operatinglife of the material.

Additionally, the coating material must be an insulator,

and it must be able to be applied to and retained by the filamentary core. Taking these various factors into consideration it is proposed to use a material for the coating 'which has a sticking potential such that in combination with the "core material of the damper fiber it provides an 'eifective sticking potential which will be considerably,

" grille potential Vg, throughout the life of the tube.

With such a composite damper fiber, the operation of lthe tube will quickly charge the damper element toward of the fiber surface tends to charge up to, but not sub-' stantially in excess of, the space potential of the region immediately surrounding it. This minimizes the distorting effect of the damper fiber on the field in the path of the electron beam or beams, making the damper fiber itself substantially invisible in an electron-optical sense, and reducing to a minimum the electron shadow of the damper fiber on the screen.

With respect to the particular materials preferred for the coating, materials which of themselves have a characteristic sticking potential in the desired range may be applied to the damper fiber core in a coating of sufiicient thickness so that the sticking potential of the core material, if different, will not substmtially affect the effective sticking potential of the composite fiber. Alternatively, a material having a substantially higher sticking potential than that of the core material may be employed, and may be applied to the core as a coating of suflficient thinness so that the composite fiber will have an effective sticking potential in the desired range.

A preferred coating material which has been found to be satisfactory is magnesium fluoride. This material may be applied, for example to a fiber core of Nonex glass, as a coating in the thickness range of preferably one to three thousand angstroms, and the resultant composite fiber will have an efiective sticking potential in the range of 7 /2 kv., which is particularly useful in color television picture tubes of the 21 inch size. Another preferred coating material is calcium fluoride. Both magnesium and calcium fluoride may be readily applied to a suitable insulating core, for example by evaporation in vacuo.

The present invention thus provides an improved grille vibration damper fiber of composite construction which has all the mechanical advantages of fibers heretofore suggested yet minimizes distortions of the electrostatic field and therefore effectively eliminates objectionable damper fiber electron shadow on the tube screen.

It will be appreciated by those skilled in the art that the invention may be carried out in various ways and may take various forms and embodiments other than those illustrative embodiments heretofore described. It is to be understood that the scope of the invention is not limited by the details of the foregoing description, but will be defined in the following claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In a cathode ray tube having means for generating at least one electron beam, a first electrode disposed in the electron beam path, and a grille of fine linear conductors disposed in the electron beam path and adapted to be maintained at a potential with respect to the first electrode such as to create an electrostatic field therebetween, a grille-conductor vibration damping element adapted to extend across the grille transverse to and in engagement with the conductors thereof, said damping element comprising a central core of insulating material, and a coating on said core consisting of a material having a secondary electron emission ratio not less than unity at a potential equal to said grille potential and equal to unity at a potential higher than said grille potential, so that the potential to which each part of the vibration damping element charges during bombardment by the electron beam substantially equals the space potential of the region immediately adjacent such respective part.

2. In a cathode ray tube having means for generating at least one electron beam, a first electrode disposed in the g r e electron beam path, and agrille of fine linear conductors disposed in the electron beam path and adapted to be maintained at a potential with respect to the first electrode such as to create an electrostatic field therebetween, a grille-conductor vibration damping element adapted to extend across the grille'transverse to and in engagement with the conductors thereof, said damping element comprising a central filamentary core of insulating material, and a coating of magnesium fluoride on said core.

3. In a cathode ray tube having means for generating at least oneelectron beam, a first electrode disposed in the electron beam path, and a grille of fine linear conductors disposed in the electron beam path and adapted to be maintained at a potential with respect to the first electrode such as to create an electrostatic field therebetween, a grille-conductor vibration damping element adapted to extend across the grille transverse to and in engagement with the conductors thereof, said damping element comprising a central filamentary core of insulating material, and a coating of magnesium fluoride having a thickness of 500 to 4000 angstroms on said core.

4. In a cathode ray tube having means for generating at least one electron beam, a first electrode disposed in the electron beam path, and a grille of fine linear con ductors disposed in the electron beam path and adapted to be maintained at a potential with respect to the first electrode such as to create an electrostatic field therebetween, a grille-conductor vibration damping element adapted to extend across the grille transverse to and in engagement with the conductors thereof, said damping element comprising a central filamentary core of insulating material, and a coating of calcium fluoride on said core.

5. In a cathode ray tube having means for generating at least one electron beam, a first electrode disposed in the electron beam path, and a grille of fine linear conductors disposed in the electron beam path and adapted to be maintained at a potential with respect to the first electrode such as to create an electrostatic field therebetween, a grille-conductor vibration damping element adapted to extend across the grille transverse to and in engagement with the conductors thereof, said damping element comprising a central filamentary core of insulating material, and a coating of calcium fluoride having a thickness of 500 to 4000 angstroms on said core.

6. A grille-conductor vibration damping element for a cathode ray tube having means for generating at least one electron beam, a screen adapted to be maintained at a beam accelerating potential and to be scanned by the beam and a grille of fine linear conductors disposed in the elec tron beam path to the screen and adapted to be main tained at a potential relative to the potential of the screen such as to create an electrostatic field therebetween, said damping element extending across the grille transverse to the conductors thereof in engagement therewith, and comprising a central filamentary core of insulating material, and a coating on said core consisting of a material having a secondary electron emission ratio-versus-voltage characteristic such as to make the effective secondary emission ratio of the damping element not less than unity at the potential of the grille.

7. In a cathode ray tube having means for generating at least one electron beam, a first electrode disposed in the electron beam path, and a grille of fine linear con ductors disposed in the electron beam path and adapted to be maintained at a potential with respect to the first electrode such as to create an electrostatic field therebetween, a grille-conductor vibration damping element adapted to extend across the grille transverse to and in engagement with the conductors thereof, said damping element comprising a central core of insulating material, and a coating on said core consisting of a material having a sticking potential not less than the potential at which the grille is adapted to be maintained.

8. A grille-conductor vibration damping element for a cathode ray tube having means for generating at least one electron beam, a screen adapted to be maintained at a beam accelerating potential and to be scanned by the beam and a grille of fine linear conductors disposed in the electron beam path to the screen and adapted to be maintained at a potential relative to the potential of the screen such as to create an electrostatic field therebetween, said damping element extending across the grille transverse to the conductors thereof in engagement therewith, and comprising a central filamentary core of insulating material, and a coating on said core consisting of a material having a sticking potential not less than 6.5 kilovolts.

ReferencesCited in the file of this patent UNITED STATES PATENTS Freeman Mar. 4, 1952 Vale Oct. 18, 1955 Blanks Dec. 20, 1955 Lawrence Jan. 3, 1956 Silverman July 31, 1956 Patterson June 18, 1957 OTHER REFERENCES RCA Review, vol. 14, December 1953, page 496. 

